Abstract

The selenisation of Cu2ZnSnS4 (CZTS) nanoparticle inks offers a potential low-cost route to the creation of Earth-abundant photovoltaic Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorber layers. This work focuses on the properties of CZTS nanoparticles fabricated under different synthesis conditions, the selenisation kinetics and the performance of CZTSSe solar cell devices made using CZTS nanoparticle inks.

Initially, CZTS nanoparticles were chemically synthesised via injection of sulphur into hot metallic precursors. Their composition, structural and optical properties were found to be sensitive to the reaction temperature, cooling rate and reaction time. For a reaction at 225 °C for 30 minutes followed by relatively slow cooling (~ 5 °C /min), it was possible to, facricate kesterite CZTS nanoparticles with an energy bandgap of 1.5 eV.

CZTS nanoparticle inks have a strong effect on the performance of CZTSSe thin film solar cells. Specifically, longer reaction time of 60 minutes increased the device efficiency by increasing the concentration of acceptor levels to 5.3×1017 cm-3 in kesterite CZTSSe. Quenching the reaction rapidly (~ 20 °C/min) introduced wurtzite crystal structure and degraded the device efficiency from 5.4 % to 2.3 %. Increasing the reaction temperature to 255 °C resulted in the highest cell efficiency of 6.3 % despite the presence of secondary phase Cu2SnS3.

In creating CZTSSe photovoltaic thin film absorber layers, high temperature selenisation of the CZTS nanoparticles plays a critical role in the formation of large grains. The results of a series of experiments indicate that the selenisation reaction is controlled by metal cation re-ordering and grain boundary migration (Avrami’s model), with a migration energy of 85.38 kJ/mol. Using a high selenium vapour pressure of 226 mbar during the selenisation process it was possible to achieve a device short circuit current density of 37.9 mA/cm2 resulting from increased carrier generation towards long wavelengths.